The present invention generally relates to the field of amplifier circuits. More particularly, the present invention relates to the field of feedback amplifier circuits.
Integrated circuit (IC) semiconductor chips are critical components found within virtually all modern electronic devices such as computers, satellites, communication systems, global positioning system (GPS) devices capable of determining the specific locations of their users on the surface of the earth, cellular phones which enable their users to communicate wirelessly with other people, different types of instrumentation, along with many consumer products, to name a few. It is appreciated that integrated circuit semiconductor chips are composed of many different types of electrical components and circuitry. For example, one type of electrical circuitry that may be utilized within integrated circuits is an amplifier circuit. It is well known by those of ordinary skill in the electrical arts that amplifier circuits may be designed and utilized for providing different functionality within different integrated circuitry.
For example, amplifier circuitry is utilized within optical communication links. One particular type of amplifier circuitry typically utilized within an optical communication link is a feedback amplifier circuit having a variable closed loop gain. One of the purposes of this type of amplifier is to provide an output voltage window based on the variation of an input current signal that can vary over a wide dynamic range. In order to handle a wide dynamic range of input current signals, the feedback amplifier circuit provides a variable amount of gain to them in order to avoid saturation with large input signals and also provides enough amplitude with smaller input signals.
FIG. 1 is a schematic diagram illustrating a conventional feedback amplifier circuit 100 having a variable closed loop gain. As mentioned above, one of the main purposes of feedback amplifier circuit 100 is to provide an output voltage based on the variation of an input current signal. Specifically, feedback amplifier circuit 100 converts an input current (Iin) 120 into an output voltage (Vo) 122. It is appreciated that as the amplitude of input current 120 fluctuates, feedback amplifier circuit 100 automatically adjusts its feedback network resistance in order to respond to a wide range of input signals.
More specifically, the feedback network of amplifier circuit 100 includes resistors 102, 104 and 106 along with series switches 108, 110 and 112. It should be appreciated that series switches 108-112 are typically implemented physically as Metal-Oxide Semiconductor Field Effect Transistors (MOSFETs). Additionally, a digital automatic gain control (AGC) circuit 114 is coupled to open and close series switches 108-112. Furthermore, digital automatic gain control circuit 114 is coupled to the output of a current amplifier 116 and resistor 106. As such, digital automatic gain control 114 compares the output voltage 122 of amplifier circuit 100 with some reference voltage and then adjusts the feedback network resistance in order to achieve an output voltage 122 that is within a certain window in response to variations of input signal 120. That is, digital automatic gain control 114 circuit is able to vary the feedback network resistance by opening and closing series switches 108-112 thereby changing the gain of amplifier circuit 100. For example, if digital automatic gain control 114 determines that the amplitude of the output voltage 122 is becoming too large, it may automatically open series switch 108 and switch 110 while closing series switch 112. In this manner, the feedback resistance of amplifier circuit 100 decreases thereby decreasing the amplitude (gain) of voltage output 122. Therefore, feedback amplifier circuit 100 is able to provide a relatively constant output voltage window 122 irrespective of the variation of input current signal 120.
It should be understood that there are some disadvantages associated with feedback amplifier circuit 100 having a variable closed loop gain. For example, one of the disadvantages is that when series switches 108-112 are physically implemented as MOSFETs, they add additional resistance and parasitics into the feedback network of amplifier circuit 100. Additionally, the resistance associated with each MOSFET varies depending on temperature, process, and gate to source voltage. As such, it is difficult to control the gain of amplifier circuit 100 when the resistance of a series switch (e.g., 112) becomes comparable with the required feedback resistance because of the MOSFET resistance variation with process, temperature, voltage, etc. Furthermore, during high frequency operation, the gain variation of amplifier circuit 100 and in turn the bandwidth variation are difficult to control. Therefore, feedback amplifier circuit 100 provides an output gain that during particular situations is difficult to control.
Accordingly, what is needed is a feedback amplifier circuit having a variable closed loop gain which is more controllable and accurate. What is needed is an amplifier in which the gain depends only on the actual feedback resistor which can be more easily controlled through the use of precision resistors. The present invention provides these advantages and others which will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of embodiments in accordance with the present invention.
Specifically, one embodiment of the present invention is an improved feedback amplifier circuit having a variable closed loop gain which avoids including switching elements in its feedback network. That is, the switching elements are not on the active feedback path (e.g., they are isolated from the active feedback path). In this manner, the gain of the amplifier circuit may be determined only by the feedback resistance which can be more easily controlled through the use of precision resistors. Additionally, by removing the switches from the feedback network, the parasitics associated with each switch may also be removed from the feedback path of the amplifier circuit. Accordingly, the bandwidth of the feedback amplifier circuit can be controlled more effectively. Specifically, the present embodiment avoids locating the switching functionality in the feedback network by changing the output stage of the feedback amplifier circuit. In other words, the switching functionality of the present embodiment is located within the output stage of the feedback amplifier circuit. Furthermore, it is appreciated that the present embodiment provides a way to isolate the unused feedback path of the feedback amplifier circuit which has a variable closed loop gain.
In yet another embodiment, the present invention includes a feedback amplifier circuit having a variable closed loop gain. The circuit includes an amplifier coupled to receive an alternating current (AC), a direct current (DC), and a reference voltage. Additionally, the circuit includes a plurality of resistors each having one end coupled to an input of the amplifier to create an active feedback path. Furthermore, the circuit includes a switching circuit coupled to an output of the amplifier and coupled to the plurality of resistors. Moreover, the circuit includes an automatic gain control circuit coupled to said output of the amplifier and coupled to the switching circuit. It is appreciated that the automatic gain control circuit is configured to cause the switching circuit to change the resistance of the active feedback path. Furthermore, the switching circuit is isolated from the active feedback path (e.g., the switching circuit is not included in the active feedback path).